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Density functional theory (DFT) is used to investigate the geometries and metal–ligand bonding in nickel complexes of bidentate phosphines, NiX2(R2P(CH2)nPR2), where X = H, CO, n = 1–3, and R = H, Me, CF3, Et, i-Pr, t-Bu, Ph, OMe, F. The net donor–acceptor properties of the phosphine ligands can be deduced from the computed frequency of the symmetric CO stretch of the Ni(CO)2(R2P(CH2)nPR2) carbonyl complexes. This frequency (in cm–1) can be estimated from the empirical expression ν(CO) = 1988 + ∑χB – 4n, where the sum is over the four substituents on the bidentate phosphine, χB is a substituent-dependent parameter, and n is the number of carbon atoms in the backbone (1 ≤ n ≤ 3). The deduced values of χB (in units of cm–1)—t-Bu (0.0), i-Pr (0.8), Et (3.0), Me (4.0), Ph (4.3), H (6.3), OMe (10.8), CF3 (17.8), and F (18.3)—are generally similar to Tolman’s electronic parameter χ derived from nickel complexes of unidentate phosphines. For the NiH2(R2P(CH2)nPR2) hydride complexes, the global minimum is a noncl...

Monodentate and bidentate phosphine supported transition metal-dihydrogen complexes were studied extensively in last three decades by using spectroscopic (IR, NMR) and structural (X-ray and neutron single crystals diffraction) characterization techniques. A major concern was to investigate various changes in physical and chemical properties of H 2 when bound to a metal center. This article emphasizes the role of the electron rich phosphin ligands in forming molecular hydrogen complexes for a large number of transition metal ions and how the bonding of the metal with the molecular hydrogen (H 2 ) affects the ultimate fate of the H−H bond. Important features of the H 2 complexes such as elongation of the H−H bond, rapid dynamics of the site exchange of H atoms in hydride/dihydrogen complexes, binding of H 2 to the coordinately unsaturated metal centers are discussed in great detail. In many cases, the results of spectroscopic and structural studies were supported by theoretical calculations. Besides these, the focus of the present article is on the phenomena of reversible release of H 2 observed in the case of certain recently reported metal-H 2 complexes which has relevance in H 2 production. Various features of the elongated H 2 complexes and reversible binding of H 2 to a metal center discussed in this article will be of great interest to inorganic and organometallic chemists in general and to inorganic-material chemists working in the area of catalysis and storage materials using H 2 . Discussions on certain new directions of metal-dihydrogen chemistry such as photochemical generation of H 2 complexes, ionic H 2 activiation, and H 2 reductive elimination from polyhydride complexes have been included (202 references).

C−H⋯H−B dihydrogen bonding in inert molecules have been explored by studying FNgCCH (Ng = Ar, Kr) complexes with BH3–NH3, BH3–NH2Me and BH3–NHMe2 at MP2/6-311++G(d,p) and MP2/aug-cc-pVDZ level of theories. The H⋯H contact distances are found to be less than 2.4 A while binding energies are in the range of 15–18 kJ mol−1. Furthermore, changes in C–H and B–H stretching frequencies authenticate to the formation of C−H⋯H−B dihydrogen bonding in these complexes. Moreover, properties of C−H⋯H−B dihydrogen bonding have also been supplemented by molecular electrostatic potential derived charge, natural population, natural bond order and atoms in molecules analysis. Strength of hydrogen bonding and dihydrogen bonding in inert complexes have been compared under identical condition and it has been found that strength of dihydrogen bonded complexes are ∼15% weaker as compared to hydrogen bonded complexes.

A new mechanism involving the formation of a dihydrogen intermediate is described using the TPSS level of the DFT method for the generation of hydrogen from water catalyzed by the PNN Ru(II) pincer complex 4 (Science2009, 324, 74). The Mulliken charge of the hydride ligand in 4 is highly negative (−1.574) and facilitates a dihydrogen bonding with one of the solvated water molecules to yield 4···H2O. In the next step, water coordinates to the metal center by forcing a conformational change in the orientations of the hydride and CO ligands to form a transient intermediate, 4′(H2O). The activation free energy, Gact for this step is 14.92 kcal/mol. Subsequently, the intermediate liberates the dihydrogen with a Gact of 10.47 kcal/mol. With respect to (4 + H2O) as reference, the effective Gact of the entire mechanism is calculated to be 32.56 kcal/mol. Unlike the previously reported mechanism, the new mechanism operates without the cooperation of the aromatization–dearomatization processes of the pincer ligand ...

In contrast to stereoselective organic chemistry, which has established sophisticated synthetic strategies to control the relative and absolute configuration at tetrahedral carbon atoms, the stereochemical control of octahedral coordination spheres is much less understood. This microreview reflects on the state of the art of asymmetric coordination chemistry of octahedral complexes within the historical context, including asymmetric coordination chemistry evolved by nature, the predetermination of metal-centered chirality with tailored chiral ligands, chiral-anion-mediated asymmetric synthesis, chiral-auxiliary-mediated asymmetric coordination chemistry, and finally, very recent work on the catalytic asymmetric synthesis of an octahedral coordination complex. The stereocontrolled synthesis of octahedral metal complexes is an important problem of contemporary coordination chemistry and will ultimately provide the necessary synthetic tools to fully exploit the opportunities provided by the rich stereochemistry of octahedral coordination geometries.

A new oxadiazole and its metal complexes have been synthesized and characterized. The ligand potassium 2-(2-phenylisonicotinoyl)hydrazinecarbodithioate [K+(H2L)−] (1) on reaction with transition metals in EtOH-H2O medium undergoes cyclization and converted to 5-(2-phenyl-4-pyridyl)-1,3,4-oxadiazole-2-thione (ppot), which yielded a series of mononuclear transition metal complexes. Four transition metal complexes of ligand are prepared and characterized based on elemental analysis, IR,1H NMR, UV-vis spectra, and thermogravimetry-differential thermal analysis (TG-DTA). From the obtained data, the structure of complexes has the general formula [M(ppot)2Cl2(H2O)2], where M Cu(II), Co(II), Ni(II), Zn(II). The thermal behaviors of these complexes showed that the complexes lost coordinated water molecules in the first step followed by decomposition of the ligand molecule in the subsequent steps. The ligand and its transition metal complexes were screened against fungi and bacteria in vitro. Data obtained showed t...

The excited-state hydrogen and dihydrogen bonding of a dihydrogen-bonded phenol–borane–dimethylamine (BDMA) complex was investigated theoretically by use of time-dependent density functional theory...

Reaction of the acidic tungsten(II) hydride 2 with the nickel(II) pincer complex 1 in either THF or toluene after an initial dihydrogen bonding (DHB) interaction led to the formation of the Ni-W bimetallic species 3 (see picture). The first example of DHB between two metal hydrides with opposite polarity was analyzed by NMR and IR spectroscopy, X-ray crystallography, and DFT calculations.

The reactions of group V metal dioxide molecules with dihydrogen have been studied by matrix isolation infrared spectroscopy. The ground state VO(2) molecule is able to cleave dihydrogen heterolytically and spontaneously in forming the HVO(OH) molecule in solid argon. In contrast, the reaction of VO(2) with dideuterium to form DVO(OD) proceeds only under UV-visible excitation via a weakly bound VO(2)(η(2)-D(2)) complex. Theoretical calculations predict that the dihydrogen cleavage process is thermodynamically exothermic with a small barrier. The niobium and tantalum dioxide molecules react with dihydrogen to give primarily the side-on bonded metal dioxide bis-dihydrogen complexes, NbO(2)(η(2)-H(2))(2) and TaO(2)(η(2)-H(2))(2), which are further transferred to the HNbO(OH) and HTaO(OH) molecules via photoisomerization in combination with H(2) elimination under UV-visible light excitation.

Ammonia borane (AB) is an attractive candidate for the chemical storage of hydrogen. Recently, our research group reported that the complex [(pocop)IrH2] (pocop= k -C6H31,3-[OP(tBu)2]2) has facilitated the rapid release of H2 from AB under mild conditions. While this result is promising, iridium is too expensive for widespread application. Our efforts to extend the catalytic chemistry of [(pocop)IrH2] to Co have led to several intriguing complexes. Direct analogues of the Ir catalyst were obtained, thereby demonstrating the unusual ligation of H2 on a Co center. Herein we report the first cobalt–dihydrogen complex, which was characterized by NMR spectroscopy and studied by means of theoretical calculations. The metalation of pocop–H with Co, achieved with a method similar to procedures developed for Ir or Ni, did not proceed in satisfactory yield. An alternative approach that makes use of an iodinated pocop ligand was pursued, as described for a similar synthesis. Complex 1 was prepared in good yield by activation of the ligand with nBuLi and the addition of CoI2·THF (Scheme 1).

The 16-electron, coordinatively unsaturated, dicationic ruthenium complex Ru(P(OH)(2)(OMe))(dppe)(2)]OTf](2) (1a) brings about the heterolysis of the C-H bond in phenylacetylene to afford the phenylacetylide complex trans-Ru(C CPh)(P(OH)(2)(OMe))(dppe)(2)]OTf] (2). The phenylacetylide complex undergoes hydrogenation to give a ruthenium hydride complex trans-Ru(H)(P(OH)(2)(OMe))(dppe)(2)]OTf] (3) and phenylacetylene via the addition of H-2 across the Ru-C bond. The 16-electron complex also reacts with HSiCl3 quite vigorously to yield a chloride complex trans-Ru(Cl)(P(OH)(2)(OMe))(dppe)(2)]OTf] (4). On the other hand, the other coordinatively unsaturated ruthenium complex Ru(P(OH)(3))(dppe)(2)]OTf](2) (1b) reacts with a base N-benzylideneaniline to afford a phosphonate complex Ru(P(O)(OH)(2))(dppe)(2)]OTf] (5) via the abstraction of one of the protons of the P(OH)(3) ligand by the base. The phenylacetylide, chloride, and the phosphonate complexes have been structurally characterized. The phosphonate complex reacts with H-2 to afford the corresponding dihydrogen complex trans-Ru(eta(2)-H-2)(P(O)(OH)(2))(dppe)(2)]OTf] (5-H2). The intact nature of the H-H bond in this species was established using variable temperature H-1 spin-lattice relaxation time measurements and the observation of a significant J(H,D) coupling in the HD isotopomer trans-Ru(eta(2)-HD)(P(O)(OH)(2))(dppe)(2)]OTf] (5-HD). (C) 2010 Elsevier B. V. All rights reserved.

Addition of the pendant amine ligand PNRP (PNRP = Et2PCH2NRCH2PEt2; R = Me, Ph, n-Bu) to Mn(CO)5Br gives fac-Mn(PNRP)(CO)3Br. Photolysis of fac-Mn(PNRP)(CO)3Br with dppm [dppm = 1,2-bis(diphenylphosphino)methane] provides mixed bis(diphosphine) complexes, trans-Mn(PNRP)(dppm)(CO)(Br). Reaction of trans-Mn(PNRP)(dppm)(CO)(Br) with LiAlH4 leads to trans-Mn(PNRP)(dppm)(CO)(H). The crystal structure of trans-Mn(PNMeP)(dppm)(CO)(H) determined by x-ray diffraction shows an unusual distortion of the Mn-H towards one C-H of the dppm ligand, resulting in an H Mn CO angle of 155(1)° and C H • • • H Mn distance of 2.10(3) A. Mn(P2PhN2Bn)(dppm)(CO)(H) [P2PhN2Bn = 1, 5-diphenyl-3,7-dibenzyl-1,5-diaza-3,7-diphosphacyclooctane] can be prepared in a similar manner; its structure has one chelate ring in a chair conformation and the second in a boat conformation. The boat-conformer ring directs the nitrogen of the ring towards the carbonyl ligand, and the N • • • C distance between one N of the P2PhN2Bn ligand and CO is 3.171(4) A, indicating a weak interaction between the N of the pendant amine and the CO ligand. Reaction of NaBArF4 (ArF = = 3,5-bis(trifluoromethyl)phenyl) with Mn(P P)(dppm)(CO)(Br) produces the cations [Mn(P P)(dppm)(CO)]+. The crystal structure of [Mn(PNMeP)(dppm)(CO)][BArF4] shows two very weak agostic interactions between C-H bonds on the phenyl ring and the Mn. The cationic complexesmore » [Mn(P P)(dppm)(CO)]+ react with H2 to form dihydrogen complexes [Mn(H2)(P P)(dppm)(CO)]+ (Keq = 1 - 90 atm-1 in fluorobenzene, for a series of different P P ligands). Similar equilibria with N2 produce [Mn(N2)(P P)(dppm)(CO)]+ (Keq generally 1-3.5 atm-1 in fluorobenzene). This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less

The activation of hydrogen by complexes [Cp′Ru(PTA)2Cl] (Cp′ = C5H5, C5Me5; PTA = 1,3,5-triaza-7-phosphaadamantane) in water was investigated in a comparative DFT study carried out using a discrete + continuum model based on a four-water-molecule cluster. As a starting point were chosen the η2-H2 dihydrogen complexes [Cp′Ru(PTA)2(η2-H2)]+ (1H2), which are formed initially upon reaction of the chloride precursors with hydrogen gas. A rationale for the experimental data, showing that the monohydrido complex [CpRu(PTA){PTA(H)}H]+ (4a) and the dihydrido complex [Cp*Ru(PTA)2(H)2]Cl (3b) are the stable products for the two systems, is proposed, together with an in-depth analysis of both ligand and solvent effects in the stability of the different species, leading to more general mechanistic implications for metal-mediated hydrogen activation in water.

Irradiation of (arene)M(CO)3 complexes (M = Cr, Mo, W; arene = various benzene derivatives) in the presence of H2 results in CO extrusion and subsequent reaction with H2. For chromium and molybdenu...

The trihydride complexes [Cp*RuH3(κ1-P-iPr2PCH2X)] [X = pyridine (Py), 2a; quinoline (Quin), 2b] have been prepared by reaction of the corresponding chloro derivatives [Cp*RuCl(κ2-P,N-iPr2PCH2X)] [X = Py (1a), Quin (1b)] with NaBH4 in methanol. Both 2a and 2b exhibit quantum-mechanical exchange coupling. The proton-transfer reactions to 2a and 2b using strong as well as weak proton donors have been experimentally and computationally studied. Density functional theory studies have been performed to analyze the stability of the proposed species, the hydrogen exchange, and the protonation pathway. The reactions with weak donors such as PhCOOH, indole, or salicylic acid in benzene or toluene result in the formation of hydrogen-bonded adducts between the proton donor and the pendant pyridine or quinoline group. However, in a more polar solvent such as dichloromethane, there is spectral evidence for the proton transfer to the hydride to yield a dihydrogen complex. The protonation with CF3SO3H in CD2Cl2 occurs i...

Progress in the hydrogen fuel field requires a clear understanding and characterization of how materials of interest interact with hydrogen. Due to the inherently quantum mechanical nature of hydrogen nuclei, any theoretical studies of these systems must be treated quantum dynamically. One class of material that has been examined in this context are dihydrogen complexes. Since their discovery by Kubas in 1984, many such complexes have been studied both experimentally and theoretically. This particular study examines the rotational dynamics of the dihydrogen ligand in the Fe(H)2(H2)(PEtPh2)3 complex, allowing for full motion in both the rotational degrees of freedom and treating the quantum dynamics (QD) explicitly. A "gas-phase" global potential energy surface is first constructed using density functional theory with the Becke, 3-parameter, Lee-Yang-Parr functional; this is followed by an exact QD calculation of the corresponding rotation/libration states. The results provide insight into the dynamical correlation of the two rotation angles as well as a comprehensive analysis of both ground- and excited-state librational tunneling splittings. The latter was computed to be 6.914 cm-1-in excellent agreement with the experimental value of 6.4 cm-1. This work represents the first full-dimensional ab initio exact QD calculation ever performed for dihydrogen ligand rotation in a coordination complex.

t-Butyl and t-pentylcalix[4]arenes bearing two 2-naphthol-1-hydrazone groups at the lower rim were synthesized, and showed excited-state intermolecular proton transfer fluorescent signal with basic anion. They are more sensitive to dihydrogen phosphate anion than to fluoride anion, although the latter has stronger basicity. Compared with t-butylcalix[4]arene bearing two 2-naphthol-1-hydrazone groups, t-pentylcalix[4]arenes derivative has a larger fluorescent difference between dihydrogen phosphate and fluoride anion. This finding may be used to analyze dihydrogen phosphate anion in the presence of fluoride anion and provide a new approach for designing fluorescence probes that are highly selective for H2PO4−.

{[Cp*Ru(CO)2]2(μ-H)}+OTf − functions as a homogeneous catalyst precursor for hydrogenation of ketones to alcohols, with hydrogenations at 1 mol % catalyst loading at 90 °C under H2 (820 psi) proceeding to completion and providing >90% yields. Hydrogenation of methyl levulinate generates γ-valerolactone, presumably by ring-closing of the initially formed alcohol with the methyl ester. Experiments in neat Et2C═O show that the catalyst loading can be

In this work, H2 activation processes in hydrogenation of ketones catalyzed by late transition metal−ligand bifunctional catalysts have been studied using the DFT method. For systems A (RuH2-diphosephine/diamine complex) and B (Ru-η5-Cp*-1,2-diamine complex), the dihydrogen activation process in neutral and basic conditions (path 1) consisted of two steps: H2 coordination and H−H cleavage. However, dihydrogen activations catalyzed by complexes C−F (Ru-η6-arene and Rh/Ir-cyclopentadiene complexes) along path 1 consist of only H−H cleavage due to the absence of H2 coordination. Thus, systems C−F have higher energy barriers (ΔG >27 kcal/mol) for dihydrogen activation than systems A and B. However, for systems C−F under acidic conditions, dihydrogen activation (path 2) consists of the two steps involving H2 coordination; thus the dihydrogen activation barriers decrease greatly, resulting in an easy splitting of H2. These results agree well with experiments. In the conversion from transfer hydrogenation to H2 ...

The ruthenium hydrido bis(dihydrogen) complex [Cn*RuH(H2)2][PF6] (2-PF6; Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane) was synthesized by treatment of Cn*RuCl3 with NaBH4. Complex 2-PF6 undergoes ...

The interaction phenomenon H−δ···H+δ between two hydrogen atoms binding each other is well-known in dihydrogen-bonded complexes. Either by experimental or theoretical viewpoint, dihydrogen bonds are often known as directional or bifurcate interactions. Regarding the beryllium hydride BeH2, its capacity to form bimolecular complexes with proton donors has been demonstrated, but in some cases, trimolecular complexes are also characterized in a minimum of the potential energy surface. As such, in this work is presented a theoretical study about the formation of trimolecular dihydrogen complexes with three hydrogen centers. By taking into account the beryllium hydride BeH2 as proton acceptor, two classical proton donors were chosen, HCN and HNC. The great goal of this work is the analysis of two dihydrogen complexes types: bifurcate BeH2···2HX and linear HX···BeH2···HX. In these systems, it is discussed the capacity of one hydride H−δ (H–Be–H−δ) to interact simultaneously with two proton donors, as well as when two hydrides H−δ (−δH–Be–H−δ) form linear dihydrogen bonds. In this context, the analysis of the vibrational harmonic spectrum at B3LYP/6-311 ++G(3d,3p) level of theory and the interpretation of the topological parameters derived from Quantum Theory of Atoms in Molecules (QTAIM) aided us to determine which is the most stable trimolecular complex, either bifurcate or linear. Moreover, quantification of charge transfer measured by the QTAIM formalism as well as by ChelpG calculations also were used with the purpose to justify infrared effects, such as red-shift and blue-shift stretch modes on donors (HCN and HNC) and acceptors (BeH2) of protons.

The article deals with a study to synthesize transition metal complexes of copper, cobalt, and iron with the ligand 6-methyl-5-arylhydrazono-2-thio-4-oxo-pyrimidine (MATOPyr). The synthesized ligand and all metal complexes were characterized by elemental analysis, XRD, SEM, FTIR, 1HNMR, UV-VIS, magnetic spectral studies, and Mossbauer measurements. The morphology of the ligand along with the complexes of all three metals was also deduced.

This article reviews published literature on the electrochemical reduction and oxidation of complexes containing the Fe 2 S 2 core characteristic of the active site of [FeFe]-hydrogenases. Correlations between reduction and oxidation potentials and molecular structure are developed and presented. In cases where the complexes have been studied with regard to their ability to catalyze the reduction of acids to give dihydrogen, the overpotentials for such catalyzed reduction are presented and an attempt is made to estimate, at least qualitatively, the efficiency of such catalysis.

Photoirradiation of a toluene solution of [ReH(CO) 3 (L)] [S. Bolano, J. Bravo, R. Carballo, S. Garcia-Fontan, U. Abram, E.M. Vazquez-Lopez, Polyhedron 18 (1999) 1431–1436] [L = 1,2-bis(diphenylphosphinoxy)ethane] in the presence of PPh n (OR) 3− n ( n = 0, 1; R = Me, Et) leads to the replacement of a CO ligand by the corresponding monodentate phosphite or phosphonite ligand to give new hydride compounds of formula [ReH(CO) 2 (L)(L′)] [L′ = P(OMe) 3 ( 1 ); P(OEt) 3 ( 2 ); PPh(OMe) 2 ( 3 ); PPh(OEt) 2 ( 4 )]. Protonation of compounds 1 – 4 in CD 2 Cl 2 , with HBF 4 .OMe 2 or with HOOCCF 3 at 193 K in a NMR tube, gave the corresponding dihydrogen complexes. When the temperature was increased from 193 to 293 K, the η 2 -H 2 ligand was replaced by OMe 2 or − OOCCF 3 groups (depending on the acid employed) to give new stable complexes and the loss of H 2 gas.

[HIPTN(3)N]Mo(N(2)) (MoN(2)) ([HIPTN(3)N](3-) = [(HIPTNCH(2)CH(2))(3)N](3-) where HIPT = 3,5-(2,4,6-i-Pr(3)C(6)H(2))(2)C(6)H(3)) reacts with dihydrogen slowly (days) at 22 degrees C to yield [HIPTN(3)N]MoH(2) (MoH(2)), a compound whose properties are most consistent with it being a dihydrogen complex of Mo(III). The intermediate in the slow reaction between MoN(2) and H(2) is proposed to be [HIPTN(3)N]Mo (Mo). In contrast, MoN(2), MoNH(3), and MoH(2) are interconverted rapidly in the presence of H(2), N(2), and NH(3), and MoH(2) is the lowest energy of the three Mo compounds. Catalytic runs with MoH(2) as a catalyst suggest that it is competent for reduction of N(2) with protons and electrons under standard conditions. [HIPTN(3)N]MoH(2) reacts rapidly with HD to yield a mixture of [HIPTN(3)N]MoH(2), [HIPTN(3)N]MoD(2), and [HIPTN(3)N]MoHD, and rapidly catalyzes H/D exchange between H(2) and D(2). MoH(2) reacts readily with ethylene, PMe(3), and CO to yield monoadducts. Reduction of dinitrogen to ammonia in the presence of 32 equiv of added hydrogen (vs Mo) is not catalytic, consistent with dihydrogen being an inhibitor of dinitrogen reduction.

The reaction of [(cymene)RuCl2](2) (1) with PCY3 was investigated using different stoichiometries and reaction conditions. Whereas a mixture of complex 1 and 2 equiv of PCy3 in methanol gave the known adduct [(cymene)Rucl(2)(PCy3)] (3) along with the phosphonium salt [PCy3(CH2OH)]Cl (4). prolonged heating of 1 and 4 equiv of PCy3 in methanol under argon resulted in the formation of complex [RuHCl(CO)(PCy3)(2)] (5). Dinuclear, chloro-bridged complexes were generated when 1 was reacted with only 1 equiv of PCy3]. In a mixture of THF and allyl alcohol. the carbonyl complex [(cymenc) Ru-(mu-cl)(3)Rucl(CO)(PCy3)](6) was formed. In dioxane, however, intramolecular C-H activation of the PCy3 ligand was observed. resulting in the formation of [(Cymene)R mu-Cl)(3)RuCl{PCy2(C6H9)}] (7). When the reaction was performed under an atmosphere of H-2,H- the dihydrogen complex [(cymene)Ru-(mu-Cl)(3)RuCl(H-2)(PCy3)] (8) could be isolated. An inert trinuclear cluster of the formula [RuCl2(PCy3)](3) (10) was formed when 1 was heated with 2 equiv of PCy3 in THF. Complexes 3, 6, 7, 8, and 10 as well as the phosphorium salt 4 were characterized by single-crystal X-ray analysis.

A bond descriptor based on the kinetic-energy density, the localized-orbital locator (LOL), is used to characterize the nature of the chemical bond in transition-metal hydride and dihydrogen complexes. Cationic complexes of the iron triad [MH3(PMe3)4]+ (M = Fe, Ru, Os) serve as model compounds for transition-metal hydrogen bonding, since these complexes not only present examples for hydride as well as dihydrogen complexes, but for certain representatives, the two different types of metal–hydrogen bonds are realized within the same molecule. Both types of ligands show characteristic LOL profiles: (3,–3) Γ attractors in close vicinity to the H-atom for hydride ligands, and (3,–3) Γ attractors located between the two atoms for a dihydrogen ligand with νΓ-values of 0.8 and 0.9, respectively. In-between structures combine elements of the hydride and dihydrogen ligands. Relativistic effects on the relative stability of various isomers for the set of model compounds have been evaluated.

A theoretical study of the molecular properties of AH···HCF3 and TH2···HCF3 (A = Li or Na and T = Be or Mg) model systems has been carried out at the B3LYP/6-311++G(3df,3pd) level of theory. Once the interactions between the proton donor (H+δ) of HCF3 and protonic hydrides (H−δ) have been produced, these systems appear, at first sight, to be dihydrogen-bonded complexes. Although a blue-shift was observed on the HC bond of HCF3 upon the formation of the BeH2···HCF3 dihydrogen complex, in the case of LiH···HCF3, MgH2···HCF3, and NaH···HCF3, however, a red-shift was detected on their HCF3 species. As far as B3LYP/6-311++G(3df,3pd) calculations are concerned, we have used the quantum theory of atoms in molecules to evaluate the molecular topography and charge density, according to which higher levels of charge transfer were computed on the red-shifted complexes. Finally, computation of ΔEC dihydrogen bond energies revealed that vibrational red-shifts are observed in more strongly bonded complexes (values for ΔEC ranging from 6.67 kJ mol−1 to 19.66 kJ mol−1), whereas blue-shifts are found in weakly bonded ones (higher value for ΔEC is 2.31 kJ mol−1). © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

The reactivity of a series of trans-Ru(P(2))(2)Cl(2) complexes with H(2) was explored. The complexes reacted with H(2) via a stepwise H(2) addition/heterolysis pathway to form the trans-[Ru(P(2))(2)(H(2))H](+) dihydrogen complexes. Some of the resulting eta(2)-H(2) complexes were surprisingly inert to substitution by water, even at concentrations as high as 55 M; however, the identity of the bidentate phosphine ligand greatly influenced the lability of the coordinated eta(2)-H(2) ligand. With less electron-donating phosphine ligands, the H(2) ligand was susceptible to substitution by H(2)O, whereas with more electron-rich phosphine ligands, the H(2) ligand was inert to substitution by water. Density functional theory (DFT) calculations of the ligand substitution reactions showed that the Ru-H(2) and Ru-H(2)O complexes are very close in energy, and therefore slight changes in the donor properties of the bidentate phosphine ligand can inhibit or promote the substitution of H(2)O for H(2).

Catalytic deoxygenation of 1,2-propanediol has been studied as a model the for deoxygenation of polyols and other biomass-derived compounds. Deoxygenation of 1,2-propanediol (1.0 M in sulfolane) catalyzed by {[Cp*Ru(CO)2]2(μ-H)}+OTf – (0.5 mol %) at 110 °C under H2 (750 psi) in the presence of HOTf (60 mM) gives n-propanol (54 %) as the major product, indicating a high selectivity for deoxygenation of the internal OH over the terminal OH of the diol. Di-n propyl ether forms through condensation of n-propanol with itself, and propylene glycol propyl ether arises from condensation of n-propanol with the starting material diol, giving a total of up to 80 % yield for deoxygenation / hydrogenation products under these conditions. The deoxygenation of 1,2-propanediol is strongly influenced by the concentration of acid, giving faster rates and proceeding to higher conversions as the concentration of HOTf is increased. There is little or no dependence of the rate on the pressure of H2. Propionaldehyde was observed as an intermediate, being formed through acid-catalyzed dehydration of 1,2-propanediol. This aldehyde is hydrogenated to n-propanol through an ionic pathway involving protonation of the aldehyde, followed by hydride transfer from the neutral hydride, Cp*Ru(CO)2H. The proposed mechanism for the deoxygenation/hydrogenation reaction involvesmore » formation of a highly acidic dihydrogen complex, [Cp*Ru(CO)2(η2-H2)]+OTf-. Regeneration of the dihydrogen complex occurs through reaction of Cp*Ru(CO)2OTf with H2. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.« less

The results of a combined spectroscopic and computational study of lanthanide hydrides with the general formula MH(x)(H(2))(y), where M = La, Ce, Pr, Nd, Sm, Eu, and Gd, x = 1-4, and y = 0-6 are reported. To understand the nature of the dihydrogen complexes formed with lanthanide metal hydride molecules, we have first identified the binary MH(x) species formed in the ablation/deposition process and then analyzed the dihydrogen supercomplexes, MH(x)(H(2))(y). Our investigation shows that the trihydrides bind dihydrogen more weakly than the dihydrides and that the interaction between the central lanthanide and the H(2) molecules occurs via a 6s electron transfer from the lanthanide to the H(2) molecules. Evidence is also presented for the SmH and EuH diatomic molecules and the tetrahydride anions in solid hydrogen.

The dehydrogenation reaction mechanisms of methane catalyzed by a ligated transition metal MH(+) (M = Ru, Rh, and Pd) have been investigated theoretically. Activation of methane by MH(+) complexes is proposed to proceed in a one-step manner via one transition state: MH(+) + CH(4) --MH(+)CH(4) --[TS] --(MCH(3)(+))H(2) --MCH(3)(+) + H(2). Both high-spin and low-spin potential energy surfaces are characterized in detail. Our calculations indicate that the ground-states species have low electron spin and a dominant 4d(n) configuration for RuH(+), RhH(+), and PdH(+), and the whole reaction proceeds on the ground-states potential energy surfaces with a spin-allowed manner. The MH(+) (M = Ru, Rh, and Pd) complexes are expected from the general energy profiles of the reaction pathways to efficiently convert methane to metal methyl, thus RuH(+), RhH(+), and PdH(+) are likely to be excellent mediators for the activity of methane. In the reactions of MH(+) with methane, the H(2) elimination from the dihydrogen complex is quite facile without barriers. The exothermicities of the reactions are close for Ru, Rh, and Pd: 11.1, 1.2, and 5.2 kcal/mol, respectively.

Five coordination complexes based on the flexible ligand 1,4-phenylenediacetic acid (H2pda), formulated as Zn(trans-pda) (1a), Mn(H2O)2(trans-pda) (1b), [M(cis-pda)(4,4′-bipy)]·H2O (M = Zn (2a), Co (2b), Cd (2c); H2pda = 1,4-phenylenediacetic acid; 4,4′-bipy = 4,4′-bipyridine), are reported. The crystal structures of the as-synthesized complexes were determined by single-crystal X-ray diffraction analyses. Complexes 1a and 1b have similar three-dimensional (3-D) network structures built by tetrahedral Zn nodes (for 1a) or octahedral Mn nodes (for 1b) and trans-pda2− bridges. Complexes 2a, 2b, and 2c possess similar two-dimensional (2-D) layered structures constructed from interconnecting metal-cis-pda2− double chains by 4,4′-bipy ligands. It is interesting that conformation of the flexible pda2− ligand is modulated in presence of the rigid 4,4′-bipy ligand. The trans-pda2− ligand was typically isolated in a single pda2− system whereas the cis-pda2− ligand was observed in a mixed pda2−/4,4′-bipy system. Furthermore, the dimensionalities of the complexes vary from 3-D (1a and 1b) to 2-D (2a, 2b, and 2c) when auxiliary 4,4′-bipy ligands were introduced. The photoluminescent studies on solid-state samples of complexes 1a, 2a and 2c indicate that the complexes exhibit intense fluorescent emissions at room temperature.

Laser-ablated iron, ruthenium, and osmium atoms react with hydrogen in excess argon, neon and pure hydrogen to produce the FeH(2) molecule, and the FeH(2)(H(2))(3), RuH(H(2))(4), RuH(2)(H(2))(4), and (H(2))MH complexes (M = Fe, Ru, Os), as identified through infrared spectra with D(2) and HD substitution. DFT frequency calculations support the assignment of absorptions observed experimentally. The FeH(2) molecule has a quintet ground state with a quasi-linear structure, and is repulsive to the addition of one more H(2) ligand: however, with three more H(2) ligands, stable triplet and singlet state FeH(2)(H(2))(3) supercomplexes can be formed. The quintet FeH(2) molecule and FeH(2)(H(2))(3) supercomplex undergo reversible near-ultraviolet photochemical rearrangement in solid neon and hydrogen. The RuH(2) molecule has a bent triplet ground state and forms the stable singlet RuH(2)(H(2))(4) supercomplex, but only the latter is observed in these experiments. In like fashion RuH has a quartet ground state and the doublet RuH(H(2))(4) complex is trapped in solid hydrogen. All three (H(2))MH complexes with lower energy than MH(3) are trapped, and no absorptions are observed for MH(3) molecules.

Kinetic Studies on the Hydrogenation of Carbon Dioxide to Formic Acid using a Rhodium Complex as Catalyst A detailed kinetic analysis has been performed on the hydrogenation of carbon dioxide with the complex [(dppp)2RhH] (dppp = 1,3-bis(diphenyl-phosphino)propane in DMSO using stopped-flow techniques. Activation parameters including the activation volume for this reaction as well as for the related reaction of the formation of the dihydrogen complex [(dppp)2RhH2]HCO2 were obtained and thus allowed the formulation of a mechanism for this reaction cycle.

Tabulating the structures and characteristic NMR properties of 17 iron complexes, 98 ruthenium complexes and 70 osmium complexes that contain dihydrogen or compressed dihydride ligands reveals a variety of trends. The H H bond lengths increase from similar Fe(II) to Ru(II) to Os(II) complexes. Iron(II) displays a narrow range of H H distances for stable complexes. Electronegative atoms Cl and O, when attached on the metal trans to the dihydrogen ligand, result in elongation of the H H bond relative to more electropositive atoms H, C, P and N. The family of cyclopentadienyl ligands also causes this elongating effect. The dihydrogen ligands with short H H distances and weak interactions with the metal, especially on iron and ruthenium are in the fast spinning regime. One exception is the biporphyrin complex of ruthenium with the side-on bridging H 2 ligand which has an H H distance of 118 pm but is in the fast spinning regime. There are some ruthenium complexes with H H distances greater than 110 pm that are in the slow motion regime and several complexes of osmium with H H distances greater than 130 pm that are in this regime. The large J HH due to quantum mechanical exchange coupling are observable for some of these osmium complexes with H H distances in the range of 140–160 pm. The dihydrogen ligands in many complexes appear to have librational motions or other motions that place them in the intermediate motion regime. New equations to correlate J HD with H H distances for ruthenium dihydrogen complexes and for osmium dihydrogen complexes are introduced here.

NMR has played a major role in the characterization of dihydrogen complexes and a number of polyhydrides have been reformulated as dihydrogen complexes on the basis of NMR data. If dihydrogen complexes remain the most widely studied class of σ-complexes, silane compounds are also well recognized as an important family of σ-complexes and more recently a few σ-borane compounds have been isolated. One important problem is the discrimination between a σ-formulation and the corresponding hydrido(silyl) or hydrido(boryl) oxidative addition product. In this review we will discuss key literature data on silane and borane complexes to illustrate the benefit gained by using multinuclear NMR spectroscopy to better define the structures and bonding modes. Our goal is also to help the reader to appreciate the limits of the method and to provide valuable insights into the problem of secondary interactions.

Structural properties of the B(+)-H2 electrostatic complex are investigated through its rotationally resolved infrared spectrum in the H-H stretch region (3905-3975 cm(-1)). The spectrum, which was obtained by monitoring B(+) photofragments while the IR wavelength was scanned, is consistent with the complex having a T-shaped structure and a vibrationally averaged intermolecular separation of 2.26 A, which decreases by 0.04 A when the H2 subunit is vibrationally excited. The H-H stretch transition of B(+)-H2 is red-shifted by 220.6 +/- 1.5 cm(-1) from that of the free H2 molecule, much more than for other dihydrogen complexes with comparable binding energies. Properties of B(+)-H2 and the related Li(+)-H2, Na(+)-H2, and Al(+)-H2 complexes are explored through ab initio calculations at the MP2/aug-cc-pVTZ level. The unusually large red-shift for B(+)-H2 is explained as due to electron donation from the H2 sigma(g) bonding orbital to the unoccupied 2p(z) orbital on the B(+) ion.

A new series of binary mononuclear complexes were prepared from the reaction of the hydrazone ligand, 2-carboxyphenylhydrazo-benzoylacetone (H2L), with the metal ions, Cd(II), Cu(II), Ni(II), Co(II), Th(IV) and UO2(VI). The binary Cu(II) complex of H2L was reacted with the ligands 1,10-phenanthroline or 2-aminopyridine to form mixed-ligand complexes. The binary complexes of Cu(II) and Ni(II) are suggested to have octahedral configurations. The Cd(II) and Co(II) complexes are suggested to have tetrahedral and/or square-planar geometries, respectively. The Th(IV) and UO2(VI) complexes are suggested to have octahedral and dodecahedral geometries, respectively. The mixed-ligand complexes have octahedral configurations. The structures of all complexes and the corresponding thermal products were elucidated by elemental analyses, conductance, IR and electronic absorption spectra, magnetic moments, 1H NMR and TG-DSC measurements as well as by mass spectroscopy. The ligand and some of the metal complexes were found to activate the enzyme pectinlyase.

Catalytic H/D-exchange reactions were studied with [Ru(dtbpmp)(η2-H2)(H)2] (1) as catalyst. Under mild reaction conditions (25–75 °C) a wide range of arenes and olefins undergo H/D exchange with [D6]benzene. A preference for protons at sp2 carbons was observed with conversions up to >90 % and significant regioselectivity in certain cases. For more reaction insights NMR-based kinetic studies were performed with naphthalene as substrate, revealing an activation energy of 15.8 kcal mol–1 for the H/D exchange at the β-position. Furthermore, the key steps of the reaction mechanism were investigated by means of DFT calculations for both model complexes (PMe2 donor sites) and real catalysts (PtBu2 donor sites). The calculations resulted in Gibb's free activation energies in the range of 10–16 kcal mol–1, indicating H/D exchange at the β-position of naphthalene to be clearly favoured over the α-position, which is in full accordance with the experimental observations.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Exposure of a series of zirconocene amido and hydrazido hydride complexes, (η5-C5Me4H)2Zr(NHR)H (R = tBu, NMe2, Me, H), to 4 atm of D2 gas at 56 °C produced isotopic exchange in both the N−H and Zr...

The study in the area of luminescent d10 complexes has been developing rapidly. The metals with d10 electronic configuration have a closed d-shell, therefore the nature of the complexes are quite different from regular transition metal complexes. The complexes show a diversity of the electronic excited states. This paper reviews the characteristic nature of the d10 metal complexes with interesting examples. Copper(I), silver(I), gold(I), nickel(0),palladium(0) and platinum(0) complexes are discussed. Luminescent copper(I) complexes are classified into two groups; the first includes the complexes bearing halogen ligands, and the second have no halogen ligands. In the second group, the complexes with phosphine and diimine type ligands are mainly discussed. As for the silver(I) and gold(I) complexes, multinuclear complexes are mostly described. For the palladium(0) and platinum(0) complexes, although a small number of works have been published so far, some interesting studies including our works have been described. In addition to the recent studies on the photophysical properties of the complexes, some applications are described in this review.

A series of new dicationic dihydrogen complexes of the type trans-$[Ru(\eta^2-H_2)(L)(dppm)_2][X]_2 (L=CH_3CN, CH_2=CHCN, C_6H_5CN, C_6H-5NC, CO; X 5 BF_4, OTf; dppm = Ph_2PCH_2PPh_2)$ have been prepared by protonating the precursor hydrides using either $HBF_4 . Et_2O or HOTf$. The variable temperature spinlattice relaxation times $(T_1, ms)$ and the H, D coupling constants of the $\eta^2-HD$ isotopomers indicate the intact nature of the H-H bond in these derivatives. The H-H distances of the dihydrogen complexes bearing trans-nitrile ligands are not significantly affected by the sterics and the electronics of the nitrile. On the other hand, the H-H distances of the dihydrogen complexes bearing either isonitrile or CO ligand are comparable to those possessing chelating diphosphine ligands of greater bite angle. Also, the thermal stabilities of these complexes are significantly less than those bearing diphosphines of greater bite angle. The trans- $[Ru(H)(CH_3CN)(dppm)_2][BF_4]$ complex has been characterized by X-ray crystallography.

Reaction of [OsCl6]2- with a pincer-type ligand, (S)-But2PCH2CH(NMe2)C3H6PBut2, afforded a chiral square-pyramidal 16-electron carbene complex, OsCl(PGP) (1), PGP = [κ4-But2PCH2CH(N(Me)CH)CHC2H4PBut2], in excellent yield. Reactions of 1 with hydrogen under different conditions resulted in the hydride and dihydrogen species OsH2Cl(PGP), [Os(H2)2(PGP)]+, and OsH3(PGP), which have been characterized by a combination of spectroscopic and DFT computational techniques.

The unimolecular isomerization mechanism from a secondary 2,1 → 3,1 propene unit promoted by the prototype zirconocene system H2SiCp2Zr+(P) (P = polymeryl chain) has been investigated by using density functional theory calculations (B3LYP) for both gas and solvent phases. The typical route occurring through β-hydrogen elimination to the metal, olefin rotation around the metal center and olefin reinsertion into the metal hydrogen bond has been calculated by using QM/MM calculations with the ONIOM model in the presence of the counterion CH 3 B(C 6 F 5 ) 3 - and compared to the alternative intramolecular reversible formation of a zirconocene allyl dihydrogen complex. Our calculations show that the alternative route remains energetically less accessible, at least for the prototype metallocene system used here.

The transfer of the hydrogen atoms from the η 2 -H 2 ligand to the cis -disposed olefin ligand in a ruthenium olefin–dihydrogen complex is discussed. It is realized that H 2 O and NEt 3 exhibit promoting effects in the catalytic hydrogenation of olefins with a couple of hydro(trispyrazolyl)borate (Tp)-supported ruthenium complexes. A reaction mechanism that accounts for the promoting effect has been proposed. A Tp-supported ruthenium solvento hydride complex TpRu(PPh 3 )(CH 3 CN)H was found to react with H 2 and R 3 SiH to form the fluxional dihydrogen–hydride, and η 2 -silane–hydride complexes, respectively. Although no stable and isolable σ-complex was formed with CH 4 , the solvento hydride complex was found to be active in catalyzing H/D reactions of CH 4 with some deuterated common organic solvents. In the catalytic CO 2 hydrogenation reactions in THF/H 2 O or alcohol, the complex TpRu(PPh 3 )(CH 3 CN)H generates the metal-ligand bifunctional catalyst TpRu(PPh 3 )(ROH)H (R = H or alkyl) which transfers the hydride and a proton from ROH to the CO 2 molecule in a concerted manner, without coordination of the latter to the metal center. Aminocyclopentadienyl ruthenium complexes, which exhibit intramolecular Ru–H⋯H–N dihydrogen-bonding interactions were synthesized and characterized. These complexes provide good models for the study of heterolytic cleavage of η 2 -H 2 ligand and its reverse-protonation of metal hydride to form dihydrogen complex. An indenyl ruthenium hydride complex was synthesized and found to be good catalyst for nitrile hydration reactions to give amides; these reactions nicely demonstrate the principle of utilizing dihydrogen bond to promote catalytic reactions.

Mixed-ligand OsCl(Tp)L(PPh3) complexes 1 [Tp = hydridotris(pyrazolyl)borate; L = P(OMe)3, P(OEt)3 and PPh(OEt)2] were prepared by allowing OsCl(Tp)(PPh3)2 to react with an excess of phosphite. Treatment of chlorocomplexes 1 with NaBH4 in ethanol afforded hydride OsH(Tp)L(PPh3) derivatives 2. Stable dihydrogen [Os(η2-H2)(Tp)L(PPh3)]BPh4 derivatives 3 were prepared by protonation of hydrides 2 with HBF4 · Et2O at −80 °C. The presence of the η2-H2 ligand is supported by short T1 min values and JHD measurements on the partially deuterated derivatives. Treatment of the hydride OsH(Tp)[P(OEt)3](PPh3) complex with the aryldiazonium salt [4-CH3C6H4N2]BF4 afforded aryldiazene [Os(4-CH3C6H4NNH)(Tp){P(OEt)3}(PPh3)]BPh4 derivative 4. Instead, aryldiazenido [Os(4-CH3C6H4N2)(Tp)[P(OEt)3](PPh3)](BF4)2 derivative 5 was obtained by reacting the hydride OsH(Tp)[P(OEt)3](PPh3) first with methyltriflate and then with aryldiazonium [4-CH3C6H4N2]BF4 salt. Spectroscopic characterisation (IR, 15N NMR) by the 15N-labelled derivative strongly supports the presence of a near-linear Os–NN–Ar aryldiazenido group. Imine [Os{η1-NHC(H)Ar}(Tp){P(OEt)3}(PPh3)]BPh4 complexes 6 and 7 (Ar = C6H5, 4-CH3C6H4) were also prepared by allowing the hydride OsH(Tp)[P(OEt)3](PPh3) to react first with methyltriflate and then with alkylazides.

The hydrido σ-dihydrogen complex RuClH(η2-H2)(PCy3)2 (2) reacts with the chlorosilanes HSiMe3-nCln (n = 1−3) to form the corresponding silyl σ-dihydrogen complexes RuCl(SiMe3-nCln)(η2-H2)(PCy3)2 (3Me3-nCln). These complexes display a 16-electron configuration, as shown by NMR, by X-ray data in the case of 3MeCl2, and by theoretical calculations. The σ-H2 ligand in 3MeCl2 has been located by X-ray diffraction, and the H−H distance of 1.05(3) A compares well with the value obtained by DFT/B3PW91 (1.073 A) as well as with the value of 1.08 ± 0.01 A derived from the measurement of the JHD coupling constant of 17.5 Hz for the deuterated isotopomer RuCl(SiMeCl2)(η2-HD)(PCy3)2. The series of model complexes RuCl(SiMe3-nCln)(η2-H2)(PMe3)2 (S3Me3-nCln) was investigated by DFT at the B3PW91 level. The most stable isomers have a structure that resembles the X-ray structure of 3MeCl2: i.e., a silyl σ-dihydrogen formulation. In the case of S3Me2Cl and S3MeCl2 a second minimum very close in energy was optimized and fo...

Treatment of RuCl2(PPh3)3 in benzene at room temperature with 2-vinylpyridine produces the vinylpyridine complex RuCl2(2-CH2CHC5H4N)(PPh3)2. In the presence of Cs2CO3, RuCl2(PPh3)3 reacts with 2-vinylpyridine at room temperature to give the cyclometalated complex RuCl(CHCHC5H4N)(CH2CHC5H4N)(PPh3), which is also produced from the reaction of RuHCl(PPh3)3 with 2-vinylpyridine. OsCl2(PPh3)3 reacts with 2-vinylpyridine/Cs2CO3 at room temperature to give the analogous cyclometalated complex OsCl(CHCHC5H4N)(CH2CHC5H4N)(PPh3). In the presence of NaBF4, the reaction produces [Os(CHCHC5H4N)(CH2CHC5H4N)(PPh3)2]BF4. The dihydrogen complex Os(H2)Cl(CHCHC5H4N)(PPh3)2 is produced from the reaction of OsHCl(PPh3)3 with 2-vinylpyridine. The mechanism of the cyclometalation reactions has been investigated by computational chemistry.